1. Field of the Invention
The invention relates to a method for controlling the access times to a system bus of a monitoring and/or control system having at least one communication module and at least one input/output module, which is connected to the communication module via the system bus for transmitting measurement and/or control signals and is intended to input and/or output measurement and/or control signals to field applications, the at least one communication module having a time control unit for controlling a transmission cycle which is constantly repeatedly carried out and has defined communication times for the communication and input/output modules which are connected to the system bus.
The invention also relates to a communication module for a monitoring and/or control system, said module having a system bus interface for connecting subordinate input/output modules and having a time control unit for controlling a transmission cycle which is constantly repeatedly carried out and has defined communication times for the communication and/or input/output modules which are connected to the system bus.
2. Description of the Related Art
In order to control the communication processes on a system bus of a monitoring and/or control system, exact synchronization and the definition of access times for individual communication and input/output modules, which are connected to the system bus, are necessary in order to avoid access collision.
DE 101 43 972 A1 describes communication control for a process control system communication bus, which uses a communication schedule to control communication occurring on the communication bus. The communication schedule prescribes when each of the devices which are coupled to the bus can communicate on the bus. A test algorithm is used to detect the presence of devices which are connected to the communication bus.
EtherCAT: Die Beckhoff-Lösung für Echtzeit Ethernet [EtherCAT: The Beckhoff solution for real-time Ethernet] in: SPS Magazin, Issue 5+6/2003, pages 42 to 45 discloses a method for synchronizing a control system with input/output modules which are connected to one another via an Ethernet field bus. Each input/output module has a clock in order to control the access time to the Ethernet field bus. A master clock determines the propagation time offset with respect to individual secondary clocks of the distributed control system. The distributed clocks are corrected on the basis of the respective propagation time differences so that its highly accurate network-wide time base is available.